Normally, a ship displays, based on information acquired from a radar device, a positioning device, an azimuth detection device and the like equipped on the ship and information acquired from an Automatic Identification System (AIS) equipped on another ship, positions, speeds, and moving directions of the ship concerned and another ship by a display module of the radar device or an Automatic Radar Plotting Aids (ARPA). The ARPA displays the position and the moving direction of another ship with respect to the information on the ship concerned by the display module, and automatically issues an alert when a danger situation, such as a possible collision with another ship, occurs.
For the traveling of the ship concerned, if coastlines and non-route ocean areas are also included in the obstacle object to be considered, information which benefits the navigation, such as nautical chart information and non-route ocean area information may also be required. Ship operators are requested to perform a check of a collision preventive action requirement, which is to check whether the ship concerned should take a collision preventive action against the obstacle object, such as another ship, a coastline, and a non-route ocean area, which can be an obstacle to the traveling of the ship concerned (hereinafter, referred to as an “obstacle candidate”). The ship operators are also required to evaluate and check the validity over selection of a course, a speed and the like for collision-prevention (hereinafter, referred to a “collision preventive action plan”). For this reason, a simulation calculation for estimating a spatial relationship between the ship concerned and the target object, such as another ship, which can be as an obstacle to the ship concerned in future is typically performed (hereinafter, referred to as “trial calculation”). The result is displayed by a display module to allow the ship operators to perform the above-described check.
A conventionally common display method for navigation is shown in FIG. 1 (hereinafter, referred to as a “normal display mode”). Here, a display example according to the heading-up method centering on a ship concerned S is shown. Normally, ship operators perform observation for safe traveling based on such a display screen or displayed information. A trial calculation for checking the collision preventive action requirement and evaluating or checking the collision preventive action plan is performed, if needed.
However, for an obstacle candidate which is located long-distance from the ship concerned (which is indicated as an image far away from the canter of the display screen), it may be difficult to accurately determine whether it is an obstacle candidate just by seeing the screen unless the ship concerned approaches closer to the obstacle candidate. On the crowded route, because many obstacle candidates are typically displayed, it is a great burden for the ship operators to find a true obstacle object quickly and accurately from the displayed information under such a situation.
In order to reduce the burden, the ARPA issues a collision alert automatically. Note that there are some errors in the speed and the moving direction of the ship concerned, the position of the obstacle candidate detected by the radar device, and the position and speed information of other ships acquired from the Automatic Identification Systems equipped on the other ships, respectively. Therefore, it is desired to provide another innovative navigation aid system in addition to the ARPA alert.
Further, the ship operators have to determine an obstacle object quicker and accurately on the course of the ship concerned from two or more images (i.e., two or more obstacle candidates) displayed on the display, and then have to determine when to perform what kind of course change and speed change are to be made against the obstacle objects. That is, an effective device for aiding such determination by the ship operators is desired.
For the art relevant to the above described technique, the art disclosed in JP 2005-289284(A) is known. A configuration of this art is shown in FIG. 2 and its flowchart is shown in FIG. 3. Based on a course and a speed of another ship and based on a course and a speed of the ship concerned, a possible area where the ship concerned will collide with another ship is calculated as a “disturbed zone.” This calculation result is displayed so that the distance and the azimuth of the disturbed zone with respect to the ship concerned and the estimated time of arrival (ETA) at the disturbed zone are comprehensible. If the disturbed zone is found, as a test traveling simulation for evading maneuver, a course change and a speed of the ship concerned are inputted, the results are then displayed, and thereby a route to evade the disturbed zone can be displayed as shown in FIG. 4.
The art disclosed in JP 1997-287976(A) performs a trial calculation for estimating positions of the ship concerned and another ship for every predetermined lapsed time when the ship concerned is traveling according to the collision preventive action plan for evading maneuver, and displays the result. Thereby, the validity of the collision preventive action plan is evaluated and checked.
As shown in FIG. 4, the display method disclosed in JP 1997-287976(A) calculates and displays positions of the ship concerned and another ship for every lapsed time t(i)=t(0)+Δt (here, i=1, 2 . . . ). Here, t(0) is a start time of the collision preventive trial, and Δt is a calculation time interval of the trial calculation positions. Normally, a display update period Td for which the display of the positions of the ship concerned and the obstacle is updated at every lapsed time t(i) is set to one to several seconds. Hereafter, the technique for displaying the trial calculation position for every lapsed time Δt as shown in FIG. 5 is referred to as a “continuous epoch display method.”
Meanwhile, the method of calculating and displaying the estimated positions of the ship concerned and obstacle in future as shown in FIG. 6 is also known. This method displays the estimated positions of the ship concerned and obstacles only during a single predetermined lapsed time after starting a collision preventive action. Hereafter, such a display method is referred to as a “single epoch display method.”
Note that the predetermined lapsed time is normally fixed to a time which is determined based on characteristic values, such as a rate of ship's turning (course change rate: degree/second) and a rate of ship speed change (knot/second) stored in advance as transcendental information which are unique to the ship. Specifically, a completion time of the turning is estimated by dividing a difference between the current course and a course set for the collision preventive action by the turning rate, and a completion time of the speed change is estimated by dividing a difference between the current ship speed and a ship speed set for the collision preventive action by the speed change rate. Whichever longer of the turning completion time or the speed change completion time is determined as the predetermined lapsed time.
The art disclosed in JP 2005-289284(A) makes easier the detection for existence of the obstacle to take collision preventive action and the creation of the collision preventive action plan, as well as the distance and azimuth of the disturbed zone and the estimated time of arrival (ETA) at the disturbed zone can be obtained. However, this art cannot grasp the spatial relationship between the ship concerned located at an intermediate position on the course and another ship. Therefore, it is not sufficient to check the validity of how much safely the sip concerned can avoid the obstacle according to the collision preventive action plan (i.e., the settings of the course and speed).
The art disclosed in JP 1997-287976(A) is a continuous epoch display technique in which dynamic relations of positions of the ship concerned and obstacle are displayed in a time-series manner. For this reason, it excels in that the relative position of the ship concerned and obstacle can be visually grasped on a screen and the lapsed time Δt and the display update period Td can be set arbitrarily.
However, it is difficult to carry out the trial calculation (simulation) before and after the time at which the ship operators should most carefully check the safety, that is, the time in future at which the ship concerned S and an obstacle object T1 are expected to approach the closest and before or after that time for acquiring information such as the distance at that time.
When setting an arbitrary future time and calculating and displaying positions of the ship concerned and the target object before and after the future time, there may be a possibility that the calculated future position of the ship concerned and its correct position deviate.